It is hypothesized that the isoenzyme, cyclooxygenase-2, is the major source of prostaglandin S2 production in patients w/systemic mastocytosis. SPECIFIC AIM: 1) The principal specific aim of this study is to determine whether a selective inhibitor of; cyclooxygenase-2 will block the overproduction of prostaglandin D2 that charaterizes systemic mastocytosis. 2) A secondary specific aim is to determine whether the overproduction of thromboxane A2 that we had recently discovered in systemic mastocytosis is reduced by a cyclooxygenase-2 inhibitor in parallel w/ the inhibition of PGD2 production. Reduction in thromboxane A2 biosynthesis by the cyclooxygenase 2 inhibitor would provide evidence that the overproduction of thromboxane A2 is not derived from platelets, which express only COX-1.